An OS has many aspects to it. Scheduling tasks is only one of the aspects. Async/await is just the interface/mechanism in dealing with the tasks, in this case cooperative tasks.
Interacting with hardware, dealing with interrupts, memory mapping/managing, task isolation, etc are all other aspects of an OS that are apart from task scheduling but still needed.
Cooperative multitasking works fine as long as the users/developers understand the limitation.
"Cooperative multitasking works fine as long as the users/developers understand the limitation."
The problem is, we basically already know that is not the case. We've got years of experience with what "cooperatively multitasked" OS looks like, and it was not a "works fine" situation. You can't understand the limitations at scale as they interact with each other far, far beyond the human mind's capacity to understand.
Interacting with hardware, dealing with interrupts, memory mapping/managing, task isolation, etc are all other aspects of an OS that are apart from task scheduling but still needed.
Cooperative multitasking works fine as long as the users/developers understand the limitation.